1. Field of the Invention
The present invention relates to a process for producing an .alpha.,.beta.-unsaturated carboxylic acid ester, and more particularly, to a process for producing an .alpha.,.beta.-unsaturated carboxylic acid ester by reacting an .alpha.,.beta.-unsaturated carboxylic acid and/or an .alpha.,.beta.-unsaturated carboxylic acid amide with an aliphatic alcohol in the presence of a solid acid as a catalyst.
An .alpha.,.beta.-unsaturated carboxylic acid ester is industrially very useful as a starting material for synthetic resins. In particular, methyl methacrylate is a starting material for poly(methyl methacrylate) which is excellent in weatherability and transparency.
2. Description of the Related Art
Heretofore, an .alpha.,.beta.-unsaturated carboxylic acid ester, for example, methyl methacrylate, has been produced by treating acetone cyanohydrin with concentrated sulfuric acid to form methacrylamide sulfate and esterifying it with methanol.
This method has been used for the industrial production, but this process has various drawbacks such as corrosion of the apparatus material with the concentrated sulfuric acid and formation of a large amount of ammonium sulfate of a low value as a by-product.
On the contrary, Japanese Patent Publication No. Sho 63-63537 (U.S. Pat. No. 4,464,539) discloses a process for producing an .alpha.,.beta.-unsaturated carboxylic acid ester from cyanohydrin without using sulfuric acid.
According to this method, an .alpha.-hydroxycarboxylic acid amide produced by hydration of the cyanohydrin is brought into contact with a first step solid acid catalyst in the presence of water, and then the resulting reaction mixture containing an .alpha.,.beta.-unsaturated carboxylic acid and/or an .alpha.,.beta.-unsaturated carboxylic acid amide is brought into contact with a second step solid acid catalyst together with an aliphatic alcohol to produce an .alpha.,.beta.-unsaturated carboxylic acid ester.
In this method, as representative solid acid catalysts, there are used a catalyst containing a phosphoric acid salt such as lanthanum phosphate, cerium phosphate and the like in the first step and a catalyst containing a phosphate or oxide of titanium or zirconium in the second step.
As a result, an .alpha.,.beta.-unsaturated carboxylic acid ester can be produced in a 80-89 mole % yield without forming ethers as by-broducts by a dehydration reaction of the aliphatic alcohol.
However, when an .alpha.,.beta.-unsaturated carboxylic acid ester is produced by the above-mentioned method, in addition to the end product, .alpha.,.beta.-unsaturated carboxylic acid ester, there are formed various by-products due to the action of the second step solid acid catalyst.
The by-products include alkylamines produced by the dehydration reaction of ammonia formed in the first and the second steps with the starting material, aliphatic alcohol, and N-alkyl .alpha.,.beta.-unsaturated carboxylic acid amides produced by the dehydration reaction of the .alpha.,.beta.-unsaturated carboxylic acid and/or .alpha.,.beta.-unsaturated carboxylic acid amide formed in the first step with the above-mentioned alkylamines and/or aliphatic alcohol.
The yield of the by-products such as alkylamines and N-alkyl .alpha.,.beta.-unsaturated carboxylic acid amides is about 2-9 mole % though it varies depending on the type of the second step solid acid catalyst, the reaction temperature and the like.
The formation of such by-products not only makes complicated the separation and purification steps for the .alpha.,.beta.-unsaturated carboxylic acid ester and the recirculation step of unreacted .alpha.,.beta.-unsaturated carboxylic acid and the like, but makes lower the economical efficiency of the process itself for producing the .alpha.,.beta.-unsaturated carboxylic acid ester depending on the economical values and demands for the alkylamines and N-alkyl .alpha.,.beta.-unsaturated carboxylic acid amides.